1. Field of the Invention
The present invention relates generally to transgenic plants. More specifically, it relates to methods and compositions for transgene expression using the maize A3 promoter.
2. Description of the Related Art
An important aspect in the production of genetically engineered crops is obtaining sufficient levels of transgene expression in the appropriate plant tissues. In this respect, the selection of promoters for directing expression of a given transgene is crucial. Promoters which are useful for plant transgene expression include those that are inducible, viral, synthetic, constitutive as described (Poszkowski et al., 1989; Odell et al., 1985), temporally regulated, spatially regulated, and spatio-temporally regulated (Chau et al., 1989).
A number of plant promoters have been described with various expression characteristics. Examples of some constitutive promoters which have been described include the CaMV 35S (Odell et al., 1985), CaMV 19S (Lawton et al., 1987), nos (Ebert et al., 1987), rice actin 1 (Wang et al., 1992; U.S. Pat. No. 5,641,876), and Adh (Walker et al., 1987).
Examples of tissue specific promoters which have been described include the lectin (Vodkin et al., 1983; Lindstrom et al., 1990), corn alcohol dehydrogenase 1 (Vogel et al., 1989; Dennis et al., 1984), corn light harvesting complex (Simpson, 1986; Bansal et al., 1992), corn heat shock protein (Odell et al., 1985; Rochester et al., 1986), pea small subunit RuBP carboxylase (Poulsen et al., 1986; Cashmore et al., 1983), Ti plasmid mannopine synthase (Langridge et al., 1989), Ti plasmid nopaline synthase (Langridge et al., 1989), petunia chalcone isomerase (Van Tunen et al., 1988), bean glycine rich protein 1 (Keller et al., 1989), truncated CaMV 35s (Odell et al., 1985), potato patatin (Wenzier et al., 1989), root cell (Conkling et al., 1990), maize zein (Reina et al., 1990; Kriz et al., 1987; Wandelt and Feix, 1989; Langridge and Feix, 1983; Reina et al., 1990), globulin-1 (Belanger and Kriz, 1991), .alpha.-tubulin, cab (Sullivan et al., 1989), PEPCase (Hudspeth & Grula, 1989), sucrose synthase (Yang & Russell, 1990), R gene complex-associated promoters (Chandler et al., 1989), and chalcone synthase promoters (Franken et al., 1991).
Inducible promoters which have been described include ABA- and turgor-inducible promoters, the promoter of the auxin-binding protein gene (Schwob et al., 1993), the UDP glucose flavonoid glycosyl-transferase gene promoter (Ralston et al., 1988); the MPI proteinase inhibitor promoter (Cordero et al., 1994), and the glyceraldehyde-3-phosphate dehydrogenase gene promoter (Kohler et al., 1995; Quigley et al., 1989; Martinez et al., 1989).
A class of genes which are expressed in an-inducible manner are glycine rich proteins. Expression of glycine rich proteins is induced by the plant hormone abscibic acid (ABA). Genes encoding glycine rich proteins have been described, for example, from maize (Didierjean et al., 1992; Gomez et al., 1988; Baysdorfer, Genbank Accession No. AF034945) sorghum (Cretin and Puigdomenech, 1990), and rice (Lee et al., Genbank Accession No. AF009411).
In addition to the use of a particular promoter, expression of transgenes can be influenced by other types of elements. In particular, introns have demonstrated the potential for enhancing transgene expression. For example, Callis et al. (1987) described an intron from the corn alcohol dehydrogenase gene which is capable of enhancing the expression of transgenes in transgenic plant cells. Similarly, Vasil et al. (1989) described an intron from the corn sucrose synthase gene having similar enhancing activity. The rice actin 1 intron, in particular, has found wide use in the enhancement of transgene expression in a number of different transgenic crops (McElroy et al., 1991). This 5' intron was identified from the first coding exon of the rice actin 1 sequence (McElroy et al., 1990a). Plant actin is encoded by a gene family present in all plant species studied to date (Meagher, 1991). In rice, there are at least eight actin-like sequences per haploid genome. Four of the rice actin coding sequences (rice actin 1, 2, 3 and 7) have been isolated and shown to differ from each other in the tissue and stage-specific abundance of their respective transcripts (Reece, 1988; McElroy et al., 1990a; Reece et at, 1990; U.S. Pat. No. 5,641,876; Genbank Accession numbers X15865, X15864, X15862, and X15863, respectively).
While the above studies have provided a number of useful tools for the generation of transgenic plants, there is still a great need in the art for novel promoter and enhancer sequences with beneficial expression characteristics. In particular, there is a need in the art for promoter-enhancer combinations which are capable of directing high-level expression of exogenous genes in transgenic crop plants. Many previously identified regulatory sequences fail to provide the levels of expression required to fully realize the benefits potentially conferred by expression of selected genes in transgenic plants. Additionally, many regulatory regions fail to demonstrate suitable or desirable expression profiles for transgene expression. There is, therefore, a great need in the art for the identification of novel sequences which can be used for the high-level expression of selected transgenes in economically important crop plants.